The present disclosure relates to an interlock switching circuit for protecting an electrical on-board power supply of an electrically drivable vehicle. In particular, the present disclosure relates to improvements in efficiency during the operation of an interlock switching circuit.
Interlock switching circuits are utilized in battery systems in order to ensure the high-voltage safety of an electrical system, such as, for example, an electrical drive train. Another known name for an interlock switching circuit is the term “pilot line”. In this case, a low-voltage conductor loop is installed through the components to be monitored. If this conductor loop is interrupted, for example, by the opening of a connection between the components, current can no longer flow through the conductor loop and this is detected by an evaluation unit. The evaluation unit can be disposed, for example, in a battery module within a battery housing. Typically in this case, so-called contactors are then opened, and so the entire high-voltage network in the vehicle is switched to a de-energized state. Current sources for generating the interlock signals have been used to an increasing extent recently in the prior art, since these have the advantage that, when a current is known, other evaluation circuits can also monitor the integrity of the interlock switching circuit. In this regard, FIG. 1 shows a typical interlock current source circuit 1, in which a current source is realized with a bipolar transistor T and an operational amplifier OP and the current direction is set by the four switches S1, S2, S3, S4. The external components are depicted in the figure by the resistor ZL within the conductor loop 2. On the input side, the operational amplifier OP is supplied by a reference voltage source URef and a voltage dropping at an ohmic resistor R which is disposed between the transistor T and the electrical ground. The disadvantage of the depicted circuit topology is the voltage drop across the current source T, which adversely affects the mode of operation in the case of low supply voltages and high-resistance loads ZL. It is also disadvantageous that a short-circuit protection for the two switches S1, S2, for example by connecting a resistor to the output in series, further increases the required supply voltage UB.